The present invention has for its aim the production of aqueous solutions of alkali metal hypochlorite and in particular sodium hypochlorite.
A common procedure used to manufacture aqueous solutions of alkali metal hypochlorite consists of reacting gaseous chlorine with an aqueous solution of alkali metal hydroxide according to the general reaction: EQU Cl.sub.2 +2NaOH.fwdarw.NaClO+NaCl+H.sub.2 O
The operation of this process is difficult because of the generally unstable nature of solutions of alkali metal hypochlorite. To avoid rapid decomposition of the solutions of alkali metal hypochlorite it is necessary to maintain continuously a substantial excess of alkali metal hydroxide in the reaction mixture. It is difficult to adhere to this condition in the case where the process is carried out as a batch process. Indeed in the carrying out of the batch process an aqueous solution of hydroxide and hypochlorite of alkali metal is made to circulate between a tank containing an aqueous solution of the alkali metal hydroxide and a reaction vessel into which chlorine is introduced; the solution in the reaction vessel is thus progressively enriched in alkali metal hypochlorite and it is, in consequence, convenient to control its composition continuously and accurately, so as to reduce the chlorine flow at the proper time and to terminate it as soon as the percentage of alkali metal hydroxide in the reaction mixture falls below a critical predetermined value.
To check the change of the pH or the alkali metal hydroxide content of the reaction mixture, it has been proposed in U.S. Pat. No. 3,199,949 filed on July 10, 1962 in the name of Clerbois et al, to follow the change of the derivative, with respect to time, of the oxidation-reduction (or redox) potential of the reaction mixture.
This known method of control has been proved accurate and reliable in the case of plants operating steadily, fed with controlled amounts of chlorine and alkali metal hydroxide. It is however poorly adapted to the control of a process for the manufacture of alkali metal hydroxide in plants operating under unsteady conditions, such as plants in which the chlorine supply undergoes uncontrollable fluctuations. This is generally the case of plants which are fed by waste chlorine derived from the liquefaction of the chlorine gas produced in cells for the electrolysis of sodium chloride brine.
Such uncontrollable fluctuations in the chlorine feed, which can sometimes be of considerable size are often the cause of corresponding variations in the derivative of the redox potential of the reaction mixture, even when the alkali metal hydroxide content of this is high. These fluctuations are likely in certain cases to act unfavourably on the operating control of the plant, for example by a sudden cessation of the chlorine flow to the reaction vessel.